This invention relates to light-conducting glass structures and more particularly to new light-conducting glass structures having a refractive index gradient in a direction transverse to the direction in which light is to advance. The invention further concerns a method for producing these light-conducting glass structures.
A light-conducting fibre of known type consists essentially of a core structure of a light-conducting substance of relatively high refractive index and a cladding layer of a light-conducting substance of relatively low refractive index covering the core structure. A flux of light incident to one end of the fibre is propagated therealong by being repeatedly reflected from the interface between the core structure and the cladding layer as described hereinafter and illustrated in FIG. 1 of the accompanying drawings.
However, a clad-type light-conducting fibre of this known character in which reflection is utilized is accompanied by the following problems. First, as the light flux, the amplitude of which is varying at a very high speed, and which has entered the fibre, advances by undergoing repeated total reflection, differences in the light path lengths of the individual light rays of the light flux occur, and, consequently, staggering or differences in phase are produced when the light flux exits from the fibre. When such differences in the light flux phase are existent, it is difficult to utilize the light-conducting fibre as a path for transmitting in a communication system, in which light signals varying at ultra high speed are employed.
Furthermore, as the incident light flux advances as it is reflected by curved interface surfaces within the fibre, the width of the light flux unavoidably and progressively increases, and, at the same time, there arise reflection losses at the aforementioned interface. These phenomena are also causes of lowering of the efficiency of photocommunication and other light-transmitting systems.
A light-conducting glass fibre in which the refractive index increases progressively from the surface toward the interior has already been proposed. The utilization of a glass in which the refractive index decreases in proportion to the square of the distance from the centreline for a light-conducting path or a part thereof has been proposed in order to overcome the above defects. (Proceedings of the IEEE, Vol. 53, pp. 2148-2149, Dec. 1965, and Uchida, Teiji: Denshi Tsushin Gakkai Soritsu 50-Shu-Nen Kinen Zenkoku Taikai Symposium, Yoko-Shu "Laser Oyo" - Hen, pp. 3-4, October 1967, (translation: (Japan) Society of Electronic Communication (Engineers) Fiftieth Anniversary National Convention Symposium Preprint "Laser Application" edition, pp. 3-4, October 1967 )).
However, light-conducting glass structures or light-conducting glass fibres having such refractive index distributions could not be realised because methods of producing such structures have heretofore been unknown.
Furthermore, so-called "gas lenses" have heretofore been known. For example, on pages 465, 466, and 467 of The Bell System Technical Journal, March Issue, 1965, it is disclosed that glass structures and other transparent structures having a refractive index distributions which decrease or increase as the square of the distance from their centre lines have a lens effect. However, structures which are transparent lens structures having such a refractive index distribution and, moreover, are practical have not heretofore been realized.
We have discovered that a glass structure having a progressively varying refractive index can be produced by causing the concentration of certain ingredients of the glass to vary with position as described fully hereinafter.